Piston Ring Material For Internal Combustion Engine

ABSTRACT

A piston ring material that when formed into a piston ring product, maintains excellent properties, and that at the production of piston ring rod, realizes excellent processability and shape stability. There is provided a piston ring material for internal combustion engine comprising, by mass, 0.5 to less than 0.7% C, 1.0% or less Si, 1.0% or less Mn, 12.0 to 16.0% Cr, 3.0% or less Mo and/or W (Mo+½W), 0.02 to 0.14% N and the balance Fe and unavoidable impurities, wherein the relationship of contained C, N and Cr satisfies the formulae: 25≦43.22C (%)+42.45N (%)−0.02Cr (%)≦40, and 0.15≦0.92C (%)+0.67N (%)−0.03Cr (%)≦0.30.

This application is a divisional of application Ser. No. 12/161,092filed on Jul. 16, 2008, which is a National Stage of InternationalApplication No. PCT/JP2007/058482 filed Apr. 19, 2007, claiming prioritybased on Japanese Patent Application No. 2006-116254, filed Apr. 20,2006, the contents of all of which are incorporated herein by referencein their entirety.

TECHNICAL FIELD

The present invention relates to a piston ring used for internalcombustion engine or a piston ring used with its sliding surface beingsubjected to nitriding treatment, and particularly to a material havingexcellent characteristics as a piston ring in addition to cold drawingor rolling workability at the production of piston ring.

BACKGROUND ART

As for piston rings for internal combustion engines, particularly,motorcar engines, use of conventional piston rings made of cast iron isgradually switched to use of so-called steel piston rings obtained byworking rod materials such as steel flat rods into rings. This is due tothe necessity to make thinner the rings or improve mechanical strengthof the rings for meeting the demands such as weight-saving, decrease offuel cost, speeding up and increase of output of internal combustionengines. Another large factor is the effect of conspicuous shortening ofsteps for production of rings.

The switching to steel piston rings has already been carried out in toprings and oil rings which are used under high load, and Si—Cr steels ormartensitic stainless steels of 11-17% Cr have been used as materials ofthe piston rings. Furthermore, there are used many of piston rings madeof the above materials which are subjected to chromium plating ornitriding treatment. Generally, piston ring materials are required tohave scuffing resistance and wear resistance, and these characteristicstend to improve with increase of the amount of carbides resulting fromincrease of amounts of Cr and C added, and with coarsening of carbides.However, carbides in such form cause deterioration of fatiguecharacteristics of piston ring materials and further cause breakage ofpiston rings when the piston ring materials are worked into rods, andthe rods are subjected to bending to form rings.

Under the circumstances, there is disclosed an invention in which apiston ring material having both fatigue characteristics and scuffingresistance after nitriding treatment is provided by adding incombination nitrogen N and carbon C to a martensitic stainless steel(Patent Document 1). Furthermore, based on the technology of adding N toa martensitic stainless steel and fining Cr carbide, it is disclosed toprovide a nitrided piston ring made of martensite steel which isexcellent in wear resistance, scuffing resistance, cracking resistanceand fatigue characteristics, by fining the nitride in the nitrided layerto allow the nitride to be present in a large amount and besides forminga microstructure where lamellar grain boundary compounds in the nitridedlayer are in fine state (Patent Document 2).

Patent Document 1: JP-A-2001-271144

Patent Document 2: JP-A-2002-030394

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

As one example of general production steps for piston ring, an ingotadjusted to a desired composition is produced, and the ingot issubjected to hot rolling, annealing, cold drawing, and cold rolling togive a desired sectional shape of piston ring, then quenching andtempering, and coiling into a shape of ring. Thereafter, the ring issubjected to a strain-removing heat treatment for removing residualstress accumulated in the material, a rough working, a pre-nitridingacid treatment, a gas nitriding, a working for removal of side nitridedlayer, and a finishing working. The resulting piston ring comprising theN-added steel is excellent in mechanical strength, particularly, wearresistance, scuffing resistance and fatigue characteristics, but areinferior in cold workability such as drawing or rolling in rod workingto require many annealing steps in the working steps, resulting in aproblem of increase in cost. Another problem is that the shape becomesunstable after the strain-removing heat treatment conducted for removalof residual stress in the material. Thus, further improvement isrequired.

The object of the present invention is to provide a piston ring materialwhich maintains excellent mechanical properties when formed into apiston ring product, is improved in cold drawing and rolling workabilityat production of piston ring rod, can reduce production cost of pistonring, and has shape stability after the strain-removing heat treatment.

Means for Solving the Problem

The inventors have conducted intensive research on the influence ofcomponents on cold workability in production of piston ring. They havefurther conducted intensive research on the influence of components onshape stability in the strain-removing heat treatment. As a result, ithas been found that extremely excellent cold workability and shapestability even after the strain-removing heat treatment can be obtainedwithout damaging the mechanical characteristics as a piston ring byseverely controlling amounts of C, Cr and N which are main constituentelements of the piston ring material and which form carbide and nitride,and furthermore by severely controlling mutual relation of the amountsof the elements. Thus, the present invention has been accomplished.

That is, the present invention relates to a piston ring material forinternal combustion engine which contains, by mass, 0.5% or more andless than 0.7% of C, 1.0% or less of Si, 1.0% or less of Mn, 12.0-16.0%of Cr, 3.0% or less of Mo and/or W (Mo+½W), and 0.02-0.14% of N, withthe balance Fe and unavoidable impurities, wherein the relationship ofthe contained C, N and Cr satisfies the formulas of 25≦43.22C (%)+42.45N(%)−0.02Cr (%)≦40 and 0.15≦0.92C (%)+0.67N (%)−0.03Cr (%)≦0.30.

Preferably, the piston ring material satisfies one or more ranges, bymass, of 0.60-0.68% of C, 0.1-1.0% of Si, 14.0-16.0% of Cr, more than1.5% and 3.0% or less (more preferably 1.6-2.5%) of Mo and/or W (Mo+½W),and 0.04-0.13% of N, or furthermore the relationship of the contained C,N and Cr satisfies the formulas of 29≦43.22C (%)+42.45N (%)−0.02Cr(%)≦35 and/or 0.18≦0.92C (%)+0.67N (%)−0.03Cr (%)≦0.30. Of course, apiston ring material which satisfies all of these requirements isdesirable. Furthermore, it is desirable to select Mo from Mo and/or W.

ADVANTAGES OF THE INVENTION

The piston ring material according to the present invention maintainsexcellent characteristics when formed into a piston ring product, andhas excellent cold workability and shape stability at production ofpiston ring rod. Therefore, the present invention contributes largely toimprovement of performance of piston ring, and simplification ofproduction steps, namely, reduction of cost.

BEST MODE FOR CARRYING OUT THE INVENTION

The feature of the present invention is as follows. Based on the findingthat particularly N which forms nitride gives a great influence oncharacteristics for C and Cr which form carbides and are mainconstituent elements of the piston ring material, a piston ring materialexcellent in cold workability at the production of piston ring and shapestability after heat treatment is provided by severely controlling thecorrelation of the above elements. The present invention will beexplained in detail below.

C is an important element in the present invention, and not only Cenhances scuffing resistance and wear resistance by forming carbides,but also a part of C forms solid solution in the base to improvestrength and fatigue characteristics. For attaining these purposes, Cmust be added in an amount of at least 0.5%. However, if the contentexceeds 0.7%, workability into rods or rings deteriorates. Especially,in the case of piston ring, since it is important to cheaply produce thepiston ring by increasing productivity, the content is specified to beless than 0.7%. Preferred range is 0.60-0.68%.

Si is added usually as a deoxidizer, but it influences behavior oftemper softening of steel, and influence of Si is important particularlyfor low alloy steels. Si is necessary for inhibiting temper softeningand enhancing heat resistance. However, if Si is added in a too largeamount, cold workability is deteriorated, and hence the upper limit ofSi is specified to be 1.2%, and it is preferably 1.0%. On the otherhand, the lower limit is preferably 0.1%.

Like Si, Mn is also an element necessary as a deoxidizer, and if it isadded in a too large amount, hot workability is deteriorated. Therefore,the upper limit of Mn is specified to be 1.0%.

As for Cr, a part of Cr bonds to C to form a carbide to enhance wearresistance, and a part of Cr forms a solid solution in the base toenhance corrosion resistance. Furthermore, since Cr increases tempersoftening resistance, it is necessary for improving heat settlingresistance of piston ring or ensuring hardenability to obtain sufficienthardness after heat treatment. When nitriding treatment is carried out,Cr has the effects to further improve scuffing resistance and wearresistance of the piston ring because of forming fine nitride in thenitrided layer. In order to obtain these effects, Cr must be added in anamount of at least 12.0%, but addition of it in a too large amountcauses decrease of thermal conductivity to promote increase of contactsurface temperature caused by sliding, thereby to damage seizingresistance and besides increase carbide or particle size to causeconspicuous deterioration of workability. Thus, the upper limit isspecified to be 16.0%. Preferred range is 14.0-16.0%.

Not only Mn and W per se bond to C to form hard carbides, but also apart of Mn and W form solid solution in the Cr carbide, and hence the Crcarbide per se is toughened, resulting in improvement of wearresistance. Furthermore, they contribute as secondary hardening elementsin tempering, and hence are effective for improving heat settlingresistance of piston ring. Moreover, a steel piston ring material isgenerally heated to 900-1100° C. for imparting wear resistance, scuffingresistance and fatigue characteristics of piston ring, and then hardenedunder rapid cooling, and thereafter tempered at a relatively hightemperature to adjust the hardness to 35-45 HRC, and Mo and W have theeffect to stabilize the hardened structure, which makes it possible toconvert the secondary carbide precipitated at 900-1100° C. to M₂₃C₆having fine particle size, whereby precipitation of M₇C₃ can beinhibited. That is, by fixing the secondary carbide formed at atemperature of 900-1100° C., it becomes possible to obtain structurewhich is stable even when the hardening temperature somewhat changes.

For alloy steels comprising C and Cr in the amounts based on those ofthe present invention, the above heating temperature of about 1000° C.is suitable for obtaining hardenability, namely, mechanicalcharacteristics. That is, if the heating temperature is too low,sufficient hardness cannot be obtained, and, on the other hand, if it istoo high, austenite particles become coarser to cause decrease oftoughness. Therefore, considering balancing of these characteristics,the above hardening temperature range should be employed. Addition of Moand W is necessary for obtaining the above effects, and particularlyadjustment of the lower limit of their amounts is important. As for thelower limit of the amounts of Mo and/or W which are controlled by theformula (Mo+½W), the amounts are preferably more than 1.5%, preferably1.6% or more. However, addition of them in too large amounts not onlyresults in increase of hard carbide and extreme increases of abrasionwear of cylinder, but also causes deterioration of workability.Therefore, the upper limit of the amounts of Mo and/or W which arecontrolled by the formula (Mo+½W) is specified to be 3.0% in the presentinvention. It is preferably 2.5%. Mo can give the similar effect in anamount which is half the amount of W and furthermore has the effect toimprove softening resistance, and can inhibit heat settling at temperingor strain-removing heat treatment. Therefore, from the point ofadvantageousness in cost and heat treatment characteristics, it isdesirable to add Mo alone without addition of W.

N is an element which is relatively stable even at high temperatures andprevents eutectic Cr carbide (M₇C₃) difficult to control its form by hotworking temperature or heat treatment temperature from crystallizationinto primary crystal austenite grain boundary. Thus, it becomes possibleto inhibit conspicuous deterioration of scuffing resistance ordeterioration of workability which is caused by the presence of coarseCr carbide. Moreover, since N has the effect to make finer the carbide,not only fatigue characteristics, but also mechanical characteristicsare improved, and tensile strength is improved without causingdeterioration of workability. For obtaining this effect, at least 0.02%of N is necessary, but if it is added in a too large amount, gas poresare formed in steel ingot and furthermore ingot making in the airbecomes difficult, resulting in increase of cost. Therefore, the amountof N is 0.14% or less. Preferred range is 0.04-0.13%.

In the piston ring material of the present invention, the remainderelement other than the above elements is Fe, but naturally unavoidableimpurities are present. Moreover, when impact stress is applied in usingas a piston ring, Ni may be added in an amount of 2.0% or less for thepurpose of improving toughness, and Cu may be added in an amount of 4%or less for the purpose of strengthening the matrix to improve heatsettling resistance.

Moreover, in order to meet the further demand for wear resistance, theremay be added at least one of V and Nb in an amount of 3.0% or less intotal. Not only V and Nb form carbide by bonding to C, but also a partof them form solid solution in the Cr carbide to strengthen the carbide.Furthermore, Al may be added in an amount of 1.5% or less for improvingthe hardness of nitrided layer formed on the surface of steel bynitriding treatment conducted for the purpose of imparting wearresistance to a piston ring made of steel. In addition, Co may be addedin an amount of 1.0% or less for improving corrosion resistance, and Bmay be added in an amount of 100 ppm or less for improving hardness ofthe matrix.

P and S which are impurity elements are preferably added in the amountsas small as possible, but in order to extremely reduce the amounts,selected expensive starting materials must be used, and, besides, highcost is required for refining by dissolution. However, in the presentinvention, they may be contained in the ranges of P≦0.1% and S≦0.1%which cause no particularly serious problems in characteristics andproduction.

In addition, the present invention provides a piston ring material whichhas not only mechanical characteristics as a piston ring product, butalso cold workability at production of piston ring rod and furthermoreshape stability after heat treatment. The greatest feature of thepresent invention resides in properly controlling the amounts of C, Crand N contained for obtaining these characteristics. The reasons will beexplained in detail below.

The cold workability of piston ring material at the production of roddepends largely upon amount or size of carbide in the steel, and isgoverned mainly by the contents of Cr and C. N is effective for finingthe carbide, and furthermore, N per se forms fine nitride to preventagglomeration and coarsening of nitride. Therefore, by controlling theamounts of the three elements within proper ranges, the workability canbe improved with maintaining mechanical characteristics as piston ringproduct. As a result of conducting a research on the optimum relationamong the amounts of C, Cr and N which can exhibit these effects to themaximum, it has been found that it can be evaluated by the relationusing factors obtained by multiple regression analysis when workabilityis taken as objective variable and C, Cr and N are taken as explanatoryvariables, and thus it is necessary to severely control the relation.

That is, it is to control the contents of C, N and Cr in the steel tosatisfy the formula: 25≦43.22C (%)+42.45N (%)−0.02Cr (%)≦40. If thisvalue is less than 25, workability is satisfactory, but mechanicalcharacteristics, wear resistance and scuffing resistance as piston ringdeteriorate, and if it is more than 40, workability deteriorates, andhence drawing and rolling at the production of rod material becomedifficult to require a plurality of annealing steps, resulting inincrease of cost. Preferred range of the value according to the aboveformula is 29-35.

In the production of piston ring, after coiling into a shape of ring, astrain-removing heat treatment for removing residual stress accumulatedin the material is carried out, and there is the problem of the shapebecoming unstable after the treatment. It has been confirmed by theinventors that the mechanism of the changing of shape is affected bymainly the amount and size of the carbide in the steel as in the case ofworkability. As a result, the inventors have clarified that by mutualcontrolling, within proper ranges, of the amounts of Cr and C which formmainly carbide, and N which makes fine the carbide, stable shape can beobtained even after the strain-removing heat treatment with maintainingthe mechanical characteristics as piston ring.

That is, it is to adjust the contents of C, Cr and N in the steel tosatisfy the mutual relation of 0.15≦0.92C (%)+0.67N (%)−0.03Cr (%)≦0.30in the relation using proper factors obtained by multiple regressionanalysis when mechanical characteristic is taken as objective variableand C, Cr and N are taken as explanatory variables as in the above case.If this value is less than 0.15, shape stability cannot be obtained, andif it is more than 0.30, mechanical characteristics as piston ringdeteriorate. Preferred range of the value according to the above formulais 0.18-0.30.

By properly adjusting C, Cr and N to satisfy the above two formulas,there can be provided a piston ring material having both characteristicsof workability and shape stability without causing deterioration ofproduct characteristics. As a result, it becomes possible to providepiston ring materials stable in characteristics.

For the piston ring material of the present invention having features incomposition, it is desirable to apply to the ingot making step a processcapable of carrying out micro-adjustment of components. For example, thedissolution step is preferably carried out with an induction furnace orvacuum furnace which causes less incorporation of impurities from theoutside. Furthermore, in the casting step, there may be usedre-dissolution method or continuous casting method which is advantageousfor avoiding segregation or presence of non-metallic inclusions inaddition to usual ingot making method.

Example 1

10 kg of ingots were prepared by high frequency vacuum inductiondissolution. These ingots were subjected to hot working to make rodmaterials of 15 mm square. Subsequently, they were subjected toannealing, and thereafter to a given hardening treatment (1000-1060° C.)and a given tempering treatment (600-640° C.) to make Samples Nos. 1-7adjusted to about 40 HRC in hardness. Values α and β (see footnotes ofTable 1) calculated from chemical composition of each sample andcontents of C, Cr and N are shown in Table 1.

TABLE 1 Sample Composition (mass %)*¹ No. C Si Mn P S Cr Mo W N Fe α*²β*³ The present 1 0.65 0.40 0.31 0.004 0.002 13.0 0.29 <0.01 0.05 Bal.29.956 0.242 invention 2 0.64 0.38 0.31 0.003 0.002 15.1 1.47 <0.01 0.09Bal. 31.179 0.196 3 0.67 0.40 0.32 0.006 0.002 15.3 1.51 <0.01 0.11 Bal.33.321 0.231 Comparative 4 0.84 0.39 0.32 0.007 0.002 14.8 1.53 <0.010.05 Bal. 38.131 0.362 Example 5 0.87 0.38 0.31 0.006 0.002 15.0 1.52<0.01 0.11 Bal. 41.971 0.424 6 0.74 0.39 0.33 0.007 0.002 14.9 0.75<0.01 0.12 Bal. 36.779 0.314 7 0.60 0.41 0.31 0.005 0.002 14.9 1.32<0.01 0.06 Bal. 28.181 0.145 *¹Al < 0.05%, Contents of other elementsare less than 0.01% *²α = 43.22C (%) + 42.45N (%) − 0.02Cr (%) *³β =0.92C (%) + 0.67N (%) − 0.03Cr (%)

For evaluation of rod workability as piston ring material, reduction ofarea (%) in tensile test was measured. JIS 14A contraction type testpieces (whole length: 110 mm, length of parallel part: 45 mm, diameterof parallel part: 7 mm, distance between gages: 35 mm) were obtainedfrom the material after annealing, and subjected to tensile test at roomtemperature to collect the values of reduction of area (%) of eachsample. In the use as piston ring, when this value is 45% or more, thereare no problems in cold drawing and rolling, and this value is employedas indication for excellent workability.

Next, for evaluation of shape stability of piston ring, the followingV-shape bending test was conducted. A test piece of 3 mm square×80 mm inlength as shown in FIG. 1 was obtained from the material adjusted to 40HRC in hardness, and markings were put on a side surface of the testpiece at an interval of 50 mm. On the assumption that the test piece isworked into a ring, the test piece was indented at a fixed speed so thatindentation reached 10 mm as shown in FIG. 2. The distance L₀ betweenthe markings (average value of upper distance L_(0A) and lower distanceL_(0B)) was measured. Then, a heat treatment simulating thestrain-removing heat treatment for removing the given residual stresswas carried out at 600° C. for 1 hour in Ar atmosphere, and thereafterthe distance L₁ between the markings was again measured. Ratio of changewas obtained from the thus obtained L₀, L₁ in accordance with thefollowing formula.

{(L₁−L₀)/L₀}×100 (%)

The inventors have confirmed that when this ratio of change at 600° C.is in the range of 0.15-0.3%, there is no problem in shape stability inthe use as piston ring. Therefore, 0.15-0.3% is employed as target valueas an indication for shape stability.

Results of evaluation of the workability (reduction of area) and shapestability (ratio of change) are shown in FIG. 3. From the results, itcan be seen that the piston ring materials of the present inventionwhich were severely adjusted in the amounts of C, N and Cr were superiorto the comparative materials in cold workability and shape stabilitybefore and after heat treatment.

Example 2

10 kg of ingots were prepared by high frequency vacuum inductiondissolution. These ingots were subjected to hot working to make rodmaterials of 15 mm square. Subsequently, they were subjected toannealing, and thereafter to a given hardening treatment (1000-1060° C.)and a given tempering treatment (600-640° C.) to make Samples Nos. 8-13adjusted to about 40 HRC in hardness. Values α and β (see footnotes ofTable 2) calculated from chemical composition and contents of C, Cr andN of each sample are shown in Table 2.

TABLE 2 Sample Composition (mass %)*¹ No. C Si Mn P S Cr Mo W N Fe α*²β*³ The present 8 0.64 0.40 0.29 0.005 0.002 15.3 1.6 <0.01 0.09 Bal.31.175 0.190 invention 9 0.67 0.41 0.31 0.005 0.002 14.7 1.7 <0.01 0.10Bal. 32.908 0.242 10 0.67 0.40 0.31 0.006 0.002 14.8 1.6 <0.01 0.11 Bal.33.331 0.246 11 0.67 0.41 0.30 0.006 0.002 15.1 1.6 <0.01 0.12 Bal.33.749 0.244 12 0.66 0.40 0.31 0.005 0.002 15.1 1.6 <0.01 0.13 Bal.33.742 0.241 13 0.66 0.40 0.31 0.005 0.002 15.1 1.6 <0.01 0.14 Bal.34.166 0.248 *¹Al < 0.05%, Contents of other elements are less than0.01% *²α = 43.22C (%) + 42.45N (%) − 0.02Cr (%) *³β = 0.92C (%) + 0.67N(%) − 0.03Cr (%)

For evaluation of workability and shape stability of piston ring, thesame tensile test and V-shape bending test as above were conducted. Theresults are shown in FIG. 4. From the results, it can be seen that thepiston ring materials of the present invention which were severelyadjusted in the amounts of C, N and Cr and contained Mo in a properamount had stably both the characteristics of cold workability and shapestability before and after heat treatment.

Example 3

2600 kg of ingots were prepared by an atmospheric dissolution furnace.These ingots were subjected to hot working to make a rod material of 15mm in diameter, which was referred to as Sample No. 14. Values α and β(see footnotes of Table 3) calculated from chemical composition andcontents of C, Cr are shown in Table 3.

TABLE 3 Sample Composition (mass %)*¹ No. C Si Mn P S Cr Mo W N Fe α*²β*³ The present 14 0.64 0.38 0.29 0.024 0.002 14.8 1.6 <0.01 0.11 Bal.32.034 0.219 invention *¹Al < 0.05%, Contents of other elements are lessthan 0.01% *²α = 43.22C (%) + 42.45N (%) − 0.02Cr (%) *³β = 0.92C (%) +0.67N (%) − 0.03Cr (%)

Successively, it was subjected to annealing, and thereafter to a givenhardening treatment (1040° C.) and a given tempering treatment (600-640°C.) to make a rod material adjusted to about 40 HRC in hardness, andcarbide distribution at longitudinal section was measured. A lightmicroscope was used for the measurement to obtain the size of carbides(in terms of diameter of equivalent circle) in 6 visual fields ofstructure which were viewed at random at 200× magnification (FIGS. 5)and 1000× magnification (FIG. 6). The viewing areas were 8×10⁻² mm²(200× magnification) and 3.2×10⁻² mm² (1000× magnification). The resultsof measurement are shown in FIG. 7 and FIG. 8. It can be seen from theresults that most of the carbides formed in the material had a size of 1μm or less and were finely distributed. In the piston ring material ofthe present invention, cold workability at the production of rodmaterial and shape stability at the strain-removing heat treatment aftercoiling were both improved by the fining of carbides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 This is a schematic view of shape of test piece used for V-shapebending test in Examples 1 and 2.

FIG. 2 This is a schematic view showing method of V-shape bending testin Examples 1 and 2.

FIG. 3 This shows the relation between cold workability (reduction ofarea) and shape stability (ratio of change) of piston ring materials ofthe present invention and the comparative example in Example 1.

FIG. 4 This shows the relation between cold workability (reduction ofarea) and shape stability (ratio of change) of piston ring materials ofthe present invention in Example 2.

FIG. 5 This is a structural photograph (200×) of the piston ringmaterial of the present invention in Example 3.

FIG. 6 This is a structural photograph (1000×) of the piston ringmaterial of the present invention in Example 3.

FIG. 7 This shows carbide size distribution (viewed at 200×magnification) of the piston ring material of the present invention inExample 3.

FIG. 8 This shows carbide size distribution (viewed at 1000×magnification) of the piston ring material of the present invention inExample 3.

DESCRIPTION OF REFERENCE NUMERALS

-   -   1. Test piece (3 mm square×80 mmL), 2. Markings, 3. Punch

1. A method of producing a rod for a piston ring for an internalcombustion engine comprising subjecting a piston ring material to colddrawing and/or cold rolling to give a desired sectional shape of thepiston ring, heating the piston ring material having the desiredsectional shape to 1000 to 1100° C., and subjecting the heated materialto quenching and subsequent tempering so as to form the rod for thepiston ring, the rod having an adjusted hardness, and the piston ringmaterial consisting of, by mass, 0.5% or more and less than 0.7% of C,1.2% or less of Si, 1.0% or less of Mn, 12.0-16.0% of Cr, Mo and/or W inan amount that satisfies the formula (Mo+½W)≦3.0%, 0.04-0.13% of N, 0.1%or less of P, and 0.1% or less of S, with the balance Fe and unavoidableimpurities, wherein the relationship of the contained C, N and Cr in thepiston ring material satisfies the formulas of 29≦43.22C (%)+42.45N(%)−0.02Cr (%)≦40 and 0.15≦0.92C (%)+0.67N (%)−0.03Cr (%)≦0.30.
 2. Themethod according to claim 1, wherein C is contained in an amount of0.60-0.68% by mass in the piston ring material.
 3. The method accordingto claim 1, wherein Cr is contained in an amount of 14.0-16.0% by massin the piston ring material.
 4. The method according to claim 1, whereinthe relationship of the contained C, Cr and N by mass % satisfies theformulas of 29≦43.22C (%)+42.45N (%) -0.02Cr (%)≦35 in the piston ringmaterial.
 5. The method according to claim 1, wherein the relationshipof the contained C, Cr and N by mass % in the piston ring materialsatisfies the formulas of 0.18≦0.92C (%)+0.67N (%)−0.03Cr (%)≦0.30. 6.The method according to claim 1, wherein Mo and/or W are containing inan amount that satisfies the formula (Mo+½W) is >1.5% to 3.0% by mass inthe piston ring material.
 7. The method according to claim 1, wherein Moand/or W are contained in an amount that satisfies the formula (Mo+½W)is 1.6-2.5% by mass in the piston ring material.
 8. The method accordingto claim 1, wherein Mo is selected from Mo and/or W in the piston ringmaterial.
 9. The method according to claim 1, wherein Si is contained inan amount of 0.1-1.0% by mass in the piston ring material.
 10. Themethod according to claim 6, wherein Mo is selected from Mo and/or W inthe piston ring material.
 11. The method according to claim 7, whereinMo is selected from Mo and/or W in the piston ring material.
 12. Amethod for producing a rod for a piston ring for an internal combustionengine comprising subjecting a piston ring material to cold drawingand/or cold rolling to give a desired sectional shape of a piston ring,heating the piston ring material having the desired sectional shape to1000 to 1000° C. and subjecting the heated material to quenching andsubsequent tempering so as to form the rod for the piston ring, the rodhaving an adjusted hardness, and the piston ring material consisting of,by mass, 0.60-0.68% of C, 0.1-1.0% of Si, 1.0% or less of Mn, 14.0-16.0%of Cr, 1.6-2.5% of Mo, and 0.04-0.13% of N, 0.1% or less of P, and 0.1%or less of S, with the balance Fe and unavoidable impurities, whereinthe relationship of the contained C, Cr and N in the piston ringmaterial satisfies the formulas of 29≦43.22C (%)+42.45N (%)−0.02Cr(%)≦35 and 0.18≦0.92C (%)+0.67N (%)−0.03Cr (%)≦0.30.
 13. A method forproducing a rod for a piston ring for an internal combustion enginecomprising subjecting a piston ring material to cold drawing and/or coldrolling to give a desired sectional shape of a piston ring, heating thepiston ring material having the desired sectional shape to 1000 to 1100°C. and subjecting the heated material to quenching and subsequenttempering so as to form the rod for the piston ring, the rod having anadjusted hardness, and the piston ring material consisting of, by mass,0.5% or more and less than 0.7% of C, 1.2% or less of Si, 1.0% or lessof Mn, 12.0-16.0% of Cr, Mo and/or W in an amount that satisfies theformula (Mo+½W)≦3.0%, 0.04-0.13% of N, 0.1% or less of P, and 0.1% orless of S, with the balance Fe and unavoidable impurities, wherein therelationship of the contained C, N and Cr in the piston ring materialsatisfies the formula of 29≦43.22C (%)+42.45N (%)−0.02Cr (%)≦40 and0.15≦0.92C (%)+0.67N (%)−0.03Cr (%)≦0.30, and wherein the material has areduction of area of 45% or more according to a tensile test.
 14. Themethod according to claim 1, wherein Mo and/or W are contained in anamount that satisfies the formula (Mo+½W) is 2.5% or less by mass in thepiston ring material.
 15. The method according to claim 14, wherein Moand/or W are contained in an amount that satisfies the formula (Mo+½W)is 1.7% or less by mass in the piston ring material.
 16. The methodaccording to claim 12, wherein Mo is contained in an amount of 1.7% orless by mass in the piston ring material.
 17. The method according toclaim 1, wherein the hardness of the rod for the piston ring is adjustedto 35 HRC or more by tempering.
 18. The method according to claim 12,wherein the hardness of the rod for the piston ring is adjusted to 35HRC or more by tempering.
 19. The method according to claim 13, whereinthe hardness of the rod for the piston ring is adjusted to 35 HRC ormore by tempering.
 20. The method according to claim 13, wherein Moand/or W are contained in an amount that satisfies the formula (Mo+½W)is 2.5% or less by mass in the piston ring material.